Must a debt be repaid at its value at the time of borrowing, or at its value at the time of repayment, given the fluctuation in exchange rates?

You must return 300 Kuwaiti Dinars to the lender, regardless of their value against other currencies. It is permissible to repay the debt in another currency if the lender agrees, provided that the exchange rate is at the rate of the day of repayment, not the day of borrowing.